1. Field of the Invention
The present invention relates to an output circuit.
2. Description of the Related Art
A conventional output circuit is described. FIG. 6 is a circuit diagram illustrating the conventional output circuit.
When an input signal voltage is applied to an input terminal IN, the input signal voltage is converted into a drain current via a PMOS transistor 50. This drain current is converted into an output voltage by output impedance at an output terminal OUT. The input signal voltage is also converted into a drain current via a PMOS transistor 51. The difference between this drain current and a current supplied by a constant current source 56 flows as a drain current of a PMOS transistor 52. This drain current becomes a drain current of an NMOS transistor 55 via a current mirror circuit formed of the PMOS transistor 52 and a PMOS transistor 53 and a current mirror circuit formed of an NMOS transistor 54 and the NMOS transistor 55. This drain current is converted into an output voltage by output impedance at the output terminal OUT. In this way, the input signal voltage appears at the output terminal OUT after amplified by both the PMOS transistor 50 and the NMOS transistor 55 connected to the output terminal OUT. This circuit has better efficiency and higher gain than a Class-A output circuit configuration of amplifying by an output PMOS transistor alone or a Class-A output circuit configuration of amplifying by an output NMOS transistor alone (see, for example, Japanese Patent Application Laid-open No. Hei 08-8654 (FIG. 2)).
The conventional output circuit can obtain a large source current, but, because the NMOS transistor 54 is saturation-connected, a gate voltage of the NMOS transistor 55 cannot rise more than the level of a threshold voltage of the NMOS transistor 54. Accordingly, there has been a problem that a high gate-source voltage cannot be obtained in the NMOS transistor 55 and hence a large sink current does not flow, resulting in an insufficient output current.